Some applications and services provided via use of a cellular communication network may be simultaneously subject to low latency requirements and high reliability (robustness) requirements.
An example of such applications and services may be illustrated in terms of mission critical MTC (machine type communication) devices (hereinafter also termed C-MTC devices).
Mission critical MTC devices are used for communication e.g. in relation to manufacturing, process industry, automotive, and medical applications. Mission critical MTC devices require higher reliability and lower latency than previously supported in legacy communication systems (e.g. communication systems operating according to standards advocated by the Third Generation Partnership Project—3GPP—of the second generation—2G (e.g. GSM—Global System for Mobile communication), third generation—3G (e.g. UMTS—Universal Mobile Telecommunication Standard), and fourth generation—4G up to Rel.12 (e.g. UMTS LTE—UMTS, Long Term Evolution)). These requirements may, for example, be needed in order to maintain robust control loop functions.
Typically, message delays over the wireless link need to be kept low as well as the round-trip time. Typical example requirements for C-MTC devices may include a maximum message delay of 20 ms and 0.001 ppm of the messages being allowed to violate the maximum message delay. In order to meet such latency requirements (for example in 4G), strict requirements may be put on the physical layer to not (or at least to a very small degree) introduce transport block errors since each retransmission adds to the message delay (e.g. 8 ms in a typical example for 4G).
In order to provide robust functionality (high reliability) the C-MTC device typically needs to support hand-over from a first set of carriers to a second set of carriers, should the second set of carriers be provided by one or more cells that can be received with higher signal power and/or lower interference than the cell(s) providing the first set of carriers. The two sets of carriers may or may not be partially overlapping, and each set of carriers may comprise one or more carriers. One or more carriers of the second set may apply a different radio access technology (RAT) than carriers of the first set. To support inter-RAT and/or inter-frequency hand-over, measurements (e.g. cell search, monitoring, power level estimation, etc.) need to be performed to determine the second set of carriers.
Therefore, there is a need for methods and arrangements that enable performing such measurements while not violation applicable low latency requirements.
An example of measurements that may need to be performed to provide high reliability is cell detection (sell search) measurements. Cell detection is typically carried out by detecting one or more synchronization signals transmitted by a network node according to a predefined pattern.
In many systems, the synchronization signals comprise a first (primary) synchronization signal and a second (secondary) synchronization signal. The first synchronization signal may typically exist in one or a few versions, essentially providing information about the existence of a cell. In a measurement procedure, the wireless communication device may typically search for this primary synchronization signal at all possible timings and, once detected, the wireless communication device knows where to look for the secondary synchronization signal, which may typically provide a cell identity (e.g., 2G: base station identity code (BSIC); 3G: scrambling code (SC); 4G: physical cell identity (PCI)) and possibly (depending on the radio access technology) further—explicit or implicit—information regarding frame timing, transmission modes, frame type, etc.
In 4G the synchronization signals comprise a Primary Synchronization Signal (PSS) and a Secondary Synchronization signal (SSS), in 3G FDD (frequency division duplex) the synchronization signals comprise a Primary Synchronization CHannel (PSCH) and a Secondary Synchronization CHannel (SSCH), and in 2G the synchronization signals comprise a Frequency Correction CHannel (FCCH) and a Synchronization CHannel (SCH).